Re: [PATCH 2/3] lib: Introduce 2 find bit api: all_is_bit_{one,zero}

Michal Nazarewicz
2015-11-20T13:57:14Z

On Thu, Nov 19 2015, Jia He wrote:
> This patch introduces 2 lightweight bit api.
> all_bit_is_zero return 1 if the bit string is all zero.
> The addr is the start address, the size is the bit size of the bit string.
> all_bit_is_one is the opposite.
>
> Signed-off-by: Jia He <hejianet@gmail.com>
> ---
> lib/find_bit.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 50 insertions(+)
>
> diff --git a/lib/find_bit.c b/lib/find_bit.c
> index 18072ea..1d56d8d 100644
> --- a/lib/find_bit.c
> +++ b/lib/find_bit.c
> @@ -131,6 +131,56 @@ unsigned long find_last_bit(const unsigned long *addr, unsigned long size)
> EXPORT_SYMBOL(find_last_bit);
> #endif
>
> +#ifndef all_bit_is_zero
> +/*
> + * return val: 1 means all bit is zero
> + */
> +unsigned int all_bit_is_zero(const unsigned long *addr, unsigned
> size)

Why does it return unsigned int, not bool?

> +{
> + unsigned long idx;
> + unsigned long mask = size;
> +
> + if (unlikely(size == 0))
> + return 1;
> +
> + if (size > BITS_PER_LONG) {
> + for (idx = 0; idx * BITS_PER_LONG < size; idx++)
> + if (addr[idx])
> + return 0;
> +
> + mask = size - (idx - 1) * BITS_PER_LONG;

Uh?

> + }
> +
> + return !(*addr & BITMAP_LAST_WORD_MASK(mask));

BITMAP_LAST_WORD_MASK takes size not mask.

> +}

The whole implementation seems weird, this seems much better to me:

unsigned int all_bit_is_zero(const unsigned long *addr, unsigned size)
{
for (; size > BITS_PER_LONG; size -= BITS_PER_LONG, ++addr)
if (*addr)
return 0;
return !size || !(*addr & BITMAP_LAST_WORD_MASK(size));
}

But to be honest I’m not entirely sure this is worth the effort.
find_next_bit and find_next_zero_bit may do a little bit more work but
some architectures have specialised optimised versions which will be
lost if all_bit_is_zero and all_bit_is_one are introduced.

> +EXPORT_SYMBOL(all_bit_is_zero);
> +#endif
> +
> +#ifndef all_bit_is_one
> +/*
> + * return val: 1 means all bit is one
> + */
> +unsigned int all_bit_is_one(const unsigned long *addr, unsigned size)
> +{
> + unsigned long idx;
> + unsigned long mask = size;
> +
> + if (unlikely(size == 0))
> + return 1;
> +
> + if (size > BITS_PER_LONG) {
> + for (idx = 0; idx * BITS_PER_LONG < size; idx++)
> + if (~addr[idx])
> + return 0;
> +
> + mask = size - (idx - 1) * BITS_PER_LONG;
> + }
> +
> + return !(~(*addr) & BITMAP_LAST_WORD_MASK(mask));
> +}
> +EXPORT_SYMBOL(all_bit_is_one);
> +#endif
> +
> #ifdef __BIG_ENDIAN
>
> /* include/linux/byteorder does not support "unsigned long" type */
> --
> 2.5.0
>

--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, ミハウ “mina86” ナザレヴイツ (o o)
ooo +--<mpn@google.com>--<xmpp:mina86@jabber.org>-----ooO--(_)--Ooo--